Particular types of radiation sensors work under the photoelectric effect. Impinging photons are converted into electrons and are integrated (collected) in sensor pixels. After a predetermined time period (e.g. a predetermined integration cycle), the collected charge is converted into a voltage (which may occur in the pixel itself, as in standard CMOS sensors), which is supplied to the output terminals of the sensor. The voltage may reach the output in analog or digital form. Pixels usually comprise a buffer amplifier (e.g. a source follower), which drives the sense lines that are connected to the pixels by suitable addressing transistors. After charge to voltage conversion is completed and the resulting signal transferred out from the pixels, the pixels are reset in order to be ready for accumulation of new charge. Floating diffusion (FD) and reset transistors may also be included for charge detection and removal of collected charge. The reset transistor usually produces thermal noise with can be reduced or removed via Correlated Double Sampling (CDS).
One of the disadvantages of standard CMOS sensors is the sequential pixel scanning, usually row by row, generating the “rolling shutter” artifact of exposure time skew (distortion of moving objects). Global shutter is a preferred method, which eliminates distortion. CCD image sensors usually work under global shutter, but they are generally more expensive than CMOS sensors and are more sensitive to saturation artifacts: when saturation is reached in a pixel, the charge may overflow to adjacent pixels, and an artifact (blooming) appears in the image. Also during charge transfer, if saturation is reached, the charges may flow into adjacent pixel columns, producing smearing artifacts. Saturation levels are determined by factors like the area, which is a constraint in compact and portable applications. CMOS are generally less sensitive to these artifacts. In order to implement global shutter in CMOS, all pixels are exposed at the same time to radiation, but an extra charge storage should be provided. The charge storages can then be read row by row, and the image does not present skew. There are examples of incorporation of global shutter to CMOS configurations, for example by transferring charge to registers. However, saturation has to be taken into account for the pixel and also for the charge storage. Some implementations include overflow barriers and doped regions which reduce blooming and remove the excess charge into the substrate, but the excess photocharges are then lost. Additionally, some of these solutions require complex gate control and additional switches, and transfer and readout times become large, making these systems less ideal for video applications and for fast image capture.